The long-term objective of this research is to inform best practices for clinical language assessment and intervention with infants raised in bilingual environments by providing much-needed information on early bilingualism. Behavioral (conditioned head turn) and event-related potential (ERP) methods will be used to investigate phoneme and word discrimination in infants from monolingual English-speaking homes before and after controlled exposure to Spanish over a one-month period (from 9.5 to 10.5 months of age). These results will be compared to those of infants being raised in monolingual Spanish environments, monolingual English-learning infants with no Spanish exposure, and infants being raised with English and Spanish input from birth. This research will explore how a second language is integrated into an emerging first language system, and whether there are differences between early second language learning and dual primary language acquisition. Individual differences in second language learning ability during infancy will also be explored using measures of primary language ability (the MacArthur-Bates Communicative Development Inventories) and nonverbal measures of problem-solving and inhibition. [unreadable] [unreadable]